The present invention relates to a method of precision calibration of magnification of a scanning microscopes with the use of test diffraction grating.
Methods of precision calibration of a magnification of scanning microscopes with the use of test diffraction gratings are known. In the existing methods a test object is positioned and oriented on a microscope stage, and a corresponding part of the test objects is scanned, with subsequent processing of the thusly obtained data. It is believed that the existing methods can be further improved.
Accordingly, it is an object of the present invention to provide method of precision calibration of magnification of a scanning microscopes with the use of test diffraction grating.
In keeping with these objects and with others which will become apparent hereinafter, one feature of present invention resides, briefly stated, in a method of precision calibration of magnification of a scanning microscope with the use of test diffraction grating, which includes the steps of positioning an orientation of a test object on a stage of microscope so that strips of a test diffraction grating are perpendicular to a directional on which a calibration is performed, scanning of a selected portion of the test object along axes X and Y; measuring values of a signal S versus coordinates X and Y in a plane of scanning and storing said values S(x, y) in a digital form as a two-dimensional digital array; transforming the two-dimensional array of signals S(x, y) into a two dimensional array S(u, v) by turning of the axes so that a direction of a new axis U is perpendicular to the strips of the grating and a direction of a new axis v coincides with the strips of the grating; line-by-line mathematical processing of the array S(u, v) including: separation from it of a one-dimensional array-line S(u) which contains a profile of an image of periodically repeating stripes of a test-object; multiplication of the line S(u) by shifting of Its copy relative to an original by an integral number of periods, clarification of a value of the shift for example by the method of least squares in accordance with a criterion of the best coincidence of the overlapping portions of the original and the copy of the line S(u), calculation of average values S(u) in the zone of overlapping as a semisum of values of the original and copy S(u) in each point of the zone; transformation of the multiplied array S(u) into an array P(w) in accordance with formula       P    ⁡          (      w      )        =            ∑              j        =        1                    N        /        2              ⁢          xe2x80x83        ⁢          {              [                              S            ⁡                          (              j              )                                *                      S            (                          j              +              w                        ]                          }            
where N is a number of centers in the multiplied array S(u), w is an argument, jxe2x88x92 is an index of summing determination of coordinates w1, w2, w3, w4 of successive maximums of the function P(w) wherein w4 greater than w3 greater than w2 greater than w1 greater than w0; determination of an average period Txe2x80x2 of the test grating in pixels for the selected line S(u) in accordance with the formula       T    xe2x80x2    =            1      n        ⁢                  ∑                  l          +          1                a            ⁢              xe2x80x83            ⁢              (                              w                          i              +              1                                -                      w            i                          )            
where n is a number of a minima in the function P(w); moving to a next line S(u) with a new value of a coordinate V and performing the same steps of the line-by-line mathematical processing for the next line; performing a standard statistic processing of obtained set of values Txe2x80x2 corresponding to various lines v with calculation of an average period Tave for all lines and a standard deviation xcex94Tave;
and calculating a magnification Mu on an image medium in accordance with a selected direction u in accordance with the formula             M      u        =                            T          ave                *        L                              T          o                *        N              ,
wherein L is width of a medium of the image in direction of calibration, T0 is an independently attested value of a pitch of the test object, N is a total number of pixels on a line along the direction U.
When the method is performed in accordance with the present invention, the calibration of magnification of the scanning microscopes is further improved.
The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.